Showing papers in "Surface Engineering in 1995"

Laser induced shock waves as surface treatment for 7075–t7351 aluminium alloy

Abstract: A novel mechanical surface treatment using laser induced shock waves has been applied to the wrought aerospace aluminium alloy 7075–T7 351 to improve its fatigue behaviour. Optimisation of the shock parametersand mechanical effects produced by the treatment have been investigated and fatigue performance is compared with that of conventionally shot peened specimens. It is shown that use of a one-dimensional Lagrangian hydrodynamic shock wave analysis code allows the attenuation of shock waves to be simulated and the depth of the plastically affected zone to be predicted. An analytical model developed to predict residual stress levels gave good agreement with experimental values. The fatigue limit of laser shocked specimens was found to be higher than that following shot peening. This large increase (22%) appeared to result from the high levels of residual stress and the greater extent of the stress field in the laser treated specimens and, most importantly, from the preservation of the surface cond...

Surface modification by laser induced shock waves

Abstract: Laser shock surface treatment is a recently developed process designed to improve component fatigue life in a manner similar to conventional treatments such as shot peening. Irradiation with a short, intense laser pulse can be used to induce plastic strains and compressive residual stresses in metallic substrates. At laser energy densities above the threshold for melting and evaporation a hot plasma can form, expansion of which will induce a shock wave in the substrate, creating plastic strains. The effect of laser induced stress waves on residual stress, micro hardness, and surface quality of a standard steel have been investigated.

Formation of compound layers on iron during gas nitriding

Abstract: Gas nitriding experiments were carried out for two series of iron specimens at 575°C: one was preheatedin air at 300°C for 1 h before nitriding, the other was nitrided directly. It was found that the pretreatment has a significant effect on the formation of the nitrides. Without preoxidation, the rate of nitriding was inhibited by a slow surface reaction such that the nitrides were formed on the iron surface inhomogeneously. With preoxidation, a homogeneous compound layer developed rapidly on the iron surface during the subsequentnitriding. The kinetics of the compound layer formation and the effect of the preoxidation on the subsequent nitriding process are discussed together with the previous findings.

OBSERVATIONS OF DEFORMATION AND FAILURE MECHANISMS IN TiN COATINGS AFTER HARDNESS INDENTATION AND SCRATCH TESTING

Abstract: A technique for examining microstructure is described which enables observation of fracture sections through hardness indentations and scratch tracks on TiN coatings, thereby clarifying the nature ...

EFFECT OF THERMAL SPRAY PROCESS SELECTION ON TRIBOLOGICAL PERFORMANCE OF WC–Co AND Al2O3–TiO2 COATINGS

Abstract: High velocity oxyfuel (HVOF), air plasma spraying (APS), and low pressure plasma spraying (LPPS) techniques were used to deposit WC-Co and APS was used to deposit Al 2 O 3 -TiO 2 coatings on ASTM 4130 steel in a study of the effect of process selection on the friction and wear of the coatings under dry sliding conditions in air. The LPPS WC-Co, which had a cobalt matrix with few brittle η-carbides (Co 3 W 3 C and Co 6 W 6 C), small and evenly dispersed WC, and high residual compressive stress, was most abrasive to Al 2 O 3 -TiO 2

Hard coatings for orthopedic implants

Abstract: Total hip replacement (THR) surgery is currently a common and successful procedure, but THR implants sometimes fail because of aseptic loosening after relatively short periods of time. Observations from retrieved femoral and acetabular THR components have shown that scoring of both bearing surfaces occurs ; scoring of the metallic femoral head components is a result of abrasion by bone cement debris embedded in the ultrahigh molecular weight polyethylene (UHMWPE) acetabular cups. To reduce wear of these bearing surfaces the feasibility of applying titanium nitride (TiN) coatings to Ti-6Al-4V femoral heads is being investigated. In vitro wear tests of TiN coated Ti-6Al-4V against UHMWPE and TiN coated Ti-6Al-4V have been performed using a pin on disc wear testing device. Early results showed that the wear is different for each wear couple. The UHMWPE has been found to wear initially by removal of residual machining marks and then by removal of thin layers of polymer. Pins consisting of TiN coated Ti-6Al-4V run against UHMWPE discs had a thin polymer transfer film and were lightly scored. Untreated Ti-6Al-4V pins under the same conditions also had a thin polymer transfer film, but were scored more severely and caused slightly more polymer wear than the TiN coated pins. When TiN coated Ti-6Al-4V pins were tested against TiN coated Ti-6Al-4V discs the character of wear differed. The predominant wear mechanism involved oxidation.

OPTIMISATION OF TiN HARD COATINGS ON PRENITRIDED LOW ALLOY STEELS

Abstract: Hardened and tempered low alloy steels were duplex treated by nitriding and TiNx coating by hollow cathode discharge. The influence of the composition and structure of the nitrided case, the mechanical treatment of the nitrided case before deposition (grinding, polishing), and the deposition parameters (sputter cleaning, nitrogen input, bias voltage) on the properties of the coating–nitrided steel composite have been investigated. It was found that the outermost nitride layer and a porous zone in the compound layer of the nitrided steel had negative effects on the adhesion,hardness, and wear behaviour of the composite. The outer compound layer can be denitrided by prolonged strong sputter cleaning. An intermediate titanium layer improved the resistance of the hard coating to adhesive failure. Higher nitrogen levels in the deposition atmosphere made it possible to form stoichiometric TiN with good mechanical properties. Best results in the scratch test, and the finest columnar structure, were achi...

DESENSITISATION OF AUSTENITIC STAINLESS STEELS USING lASER SURFACE MELTING

Abstract: Sensitised types 304 and 316 stainless steels were laser surface melted using a Nd–YAG laser of 300 W power and 9 ms pulse width at a traverse speed of 2.5 mm s–l. Pulse energy, pulse repetition rate, and gas atmosphere were varied to give 24 different laser melting conditions. The melted specimens were tested according to ASTM A262 practice A (electrolytic etch test), and ASTM A262 practice E (immersion for 24 h in boiling Cu–CuS04 solution followed by a Ubend test) in order to assess the intergranular corrosion (IGC) resistance. The micro hardness values across the unmelted/melted region were measured and impact tests, with sensitised subsize specimens laser surface melted at the notch, were carried out. The results indicate that on lasersurface melting the sensitised microstructure disappeared and also that the specimens possessed higher IGC resistance. This is attributed to the creation of a desensitised microstructure of 200 μm thickness with dendritic–cellular structure and a heat affected z...

Jet vapor deposition™: an alternative to electrodeposition

Abstract: Electrodeposition is widely used to deposit a variety of metals and alloys. Although high rates can be achievedusing this technique, there is limited control over the microstructure of the deposits and in many instances the use of toxic precursors and disposal of toxic effluents has to be considered. This has spurred the development of alternative deposition processes. The Jet Vapor Deposition™ (JVD™) process is a rapidly emerging ‘green’ technology which allows control of microstructure at high deposition rates. A brief discussionof the principles of JVD and representative applications in the deposition of metals (such as copper and gold) and alloys (Cu–Ag and Ag–Sn) are presented.

Surface modification of polymers by low temperature plasma techniques

Abstract: Surface modification of polymers has been investigated using low temperature plasma techniques such as plasma grafting and plasma immersion ion implantation. The polymers studied include polyethylene terephthalate tyre cord, silk fabrics, and polyimide films and the changes in molecular structure of the polymer surface were analysed with the aid of X-ray photoelectron spectroscopy and attenuated total reflection infrared spectroscopy.

Process modelling of thermal spray deposition of thermosets

Abstract: Optimisation of thermal spraying, as for generic polymers, produces thermosets in the uncured condition with unsatisfactory engineering properties. The present paper develops a theoretical process modelfor thermoset reactive deposition, based on heat transfer mechanisms, which enables temperature profiles to be generated as a function of process variables. The model predicts that crosslinking can not take place in in flight precursor particles, isolated splats, or deposits without substrate preheating. It shows that the difficulty in achieving satisfactory properties is a result of the rapid heat flow to the substrate, which suppresses the crosslinking reaction. The model is applied to identify methods of enhancing the engineering properties of thermosets and indicates that substrate preheating, ceramic undercoats, and rapid curing agents can promote the curing reaction during thermal spraying. An experimental technique to evaluate the degree of crosslinking in the deposits from enthalpy determina...

FORMATION OF RESIDUAL STRESSES IN COMPOUND lAYER DURING GAS NITRIDING MEASURED BY IN SITU TECHNIQUE

Abstract: Residual stresses have a strong influence on the properties of nitrided parts. Therefore knowledge of their origin and magnitude is of great interest and aprerequisite if changes in the stresses are intended. From the relevant theories of residual stresses in gas nitrided materials it can be concluded that stresses which are generated during nitriding can be reduced by plastic deformation and increased during cooling from nitriding temperature to room temperature. The increase in residual stresses should be a result of the volume misfit generated by different thermal expansion coefficients of the compound layer and the steel matrix. The theories mentioned have not thus far been confirmed by experimental work. Therefore one aim of the investigations was to clarify the origin of residual stresses as well as the different influences on the formation of residual stresses, especially during cooling from nitriding temperature to room temperature. Previously, residual stresses of nitrided parts could onl...

LASER SURFACE ALLOYING OF AI–12Si

Abstract: An Al–12Si alloy has been laser alloyed using premixed powders and a carbon dioxide laser. Tracks 6 mm wide were alloyed with various amounts of Si, Ni, Fe, Cu, Mn, Cr, Co, Mo, and Ti and assessed for integrity,uniformity, hardness, and response to age hardening. In general, above a certain concentration the tracks were smooth with high integrity. Alloys containing more than 30%Fe cracked, and Mo and Ti were difficult to melt. Aging at 150aC for 25 h produced a significant increase in the hardness of Cu, Mn, and Cr alloyed tracks. Coatings 18 mm wide were alloyed with a series of Al–Ni and Al–Fe compositions containing various amounts of Cu, Mn, and Cr. After aging, the coatings were characterised and their resistance to cavitation erosion and sliding wear was determined. Additions of Cu, Mn, and Cr considerably increased the hardness of the Ni based coatings. In general, thecoatings had excellent properties. The resistance to erosion was very high, the best coating having an erosion rate 0.03 ti...

Abnormal nitriding behaviour of a high chromium, high manganese austenitic steel

Abstract: The nitriding rate of a high chromium, high manganese austenitic steel has been found to exhibit abnormalbehaviour similar to that of austenitic stainless steels, i.e. a sudden decrease of the rate at about 870 K; marked acceleration between 670 and 870 K; a discontinuity at about 670 K; and formation of a nitrogen supersaturated nitrided layer below 670 K. These results require a new explanation, since previous theories address the abnormalities in austenitic stainless steels only. Accelerated nitriding is discussed with reference to the free energy of formation of chromium nitrides (Cr2N and CrN) on a T–∆G° diagram. The sudden decrease of the rate is attributed to the termination of accelerated nitriding.

Influence of dielectric temperature in electrical discharge machining of hot work tool steel

Abstract: Influence of dielectric and temperature in electrical discharge machining of hot work tool steel

Thick thermal barrier coatings for diesel engines

Abstract: Application of thermal barrier coatings (TBCs) in diesel engines offers benefits through improved component lifetimes, better thermal efficiency, and lower environmental emissions. Work done in this area within COST 501 is summarised and proposals to improve coating quality through enhanced control of the plasma spraying process are put forward. The control system will use fuzzy logic to optimise spraying parameters forindividual components. Of great importance is the cooling profile undergone by the component after coating, which must be designed to provide residual strains that will enhance coating lifetime.

Lifetime determining factors during thermal cycling of zirconia based thermal barrier coatings

Abstract: Plasma sprayed yttria stabilised zirconia (YSZ) coatings were investigated to assess the factors influencing their durability during thermal cycling. For any given powder, the best performance was found to be achieved at an optimum plasma arc current, all other spray parameters being held constant. The YSZ overlayer thickness was found to be an important lifetime determining factor. Use of a NiCoCrAlY bond coat instead of Ni–Cr led to a substantial improvement in coating lifetime, with the enhancement provided by NiCoCr AlY becoming more pronounced with increasing porosity level of the ceramic overlayer. A post-coating heat treatment was also found to be beneficial to coating longevity. The relative ranking of magnesium zirconate and YSZ coatings was found to depend upon the thermal cycle adopted during testing, which has important implications in designing accelerated tests to evaluate coating performance.

Application of electric arc spraying technique to enhance corrosion resistance of steel structures on ships

Abstract: Steel structures on ships operate in hostile environments and can be subject to severe corrosion. Composite coatings consisting of aluminium arc sprayed nd organic layers can effectively increase the corrosion lifetime of such structures. The composite layer developed consists of an electric arc sprayed aluminium layer followed by a layer of organic paint to seal the aluminium coating, followed by a layer of a conventional ship's paint.

Plasma nitrocarburising with nitrogen, hydrogen, and hydrogen sulphide gas mixtures

Abstract: When a sulphur bearing gas is added to the nitrogen– hydrogen atmosphere used in a plasma thermochemical treatment, a nitrocarburising process can be performed because of the contribution from the carbon in the steel. In the present work, H2S has been used to introduce sulphur species into the discharge atmosphere. The sulphur promotes the nucleation of the 8 phase. A compound layer of thickness 30–45 μm with a duplex structure can be produced by this procedure on plain carbon steel and low alloy steel substrates. The outer zone (10–12 μm) is pure e phase and the innermost zone consists essentially of γ’ phase. The influence of the gas composition, temperature, and duration of the treatment on the microhardness profiles and thickness of the compound layer has been investigated. Wear tests, using a plain carbon steel substrate, revealed significantly better wear behaviour of the sulphur bearing nitrocarburised specimens, compared with materials treated by conventional plasma nitriding.

Fibre Optic Nd–YAG laser cladding of preplaced hastelloy C powder

Abstract: A series of experiments has been carried out to investigate the fibre optic Nd–YAG laser cladding of C–Mn steel with preplaced Hastelloy C powder. The beam from a pulsed Nd–YAG laser, operating at an average output power of 300 Jt: was transmitted to the workpiece via a step index glass fibre of length 5 m and core diameter 600 μm. A range of laser pulse energies, durations, and repetition rates was investigated forseveral focal length positions to determine their effect on clad layer formation and integrity. It was found that, for a limited range of process parameters, metallurgically sound clad layers up to 500 μm thick could be formed with less than 10% dilution by the substrate.

HARDFACING: NOT MERELY A REClAMATION PROCESS

Abstract: Billions of pounds are lost every year in industry as a result of worn parts and wastage of materials. In view of the need for energy and materials conservation, the hardfacing of parts or components must be considered as a production process from the design stage, rather than merely a reclamation process. By applying hardfaced coatings to parts or components subjected to breakdown through wear, their working lifetime can be extended by up to 5 times. The present paper outlines the types of wear and choice of hardfacing alloy, as well as spraying and welding deposition processes.

Optimisation of continuous plasma nitride-reactive magnetron deposition process for titanium nitriding of M2 steel

Abstract: A number of M2 steel substrates have been nitrided in a capacitively coupled rf plasma and TiN coated via reactive magnetron sputtering in a continuous (two stage) process Both nitrided only and fully processed substrates have been characterised by hardness and scratch adhesion testing, scanning electron microscopy,X-ray diffraction, surface profilometry, and Auger electron spectroscopy Using these techniques to identify important parameters, optimisation of the process has been made possible to give increased hardness and adhesion

Effect of metal vapour vacuum arc ion implantation of aluminium on sliding wear characteristics of Ti-6Al-4V

Abstract: The metal vapour vacuum arc ion source is introduced and its more important features and system characteristics are described. Specific reference is made to applications in the field of metallurgical surface treatment by metal ion implantation and some recent developments are reviewed. The effects of aluminium ion implantation (doses up to 1·3 × 1017 ions/cm2) onthe sliding wear properties of Ti-6Al-4 V forms the emphasis of the ensuing investigation. Damage induced in sliding wear tests was reduced by about 50% after aluminium implantation, owing to a higher surface hardness. Several techniques of surface analysis including glancing angle X-ray diffraction and glow discharge optical emission spectroscopy indicated that subsurface precipitates were primarily responsible for the observed wear mechanism. The strengthening of subsurface material delayed the onset of superficial plastic deformation and resulted in enhanced tribological properties.

Coating thickness distribution and morphology of pulsed current copper electrodeposits

Abstract: Coating thickness distribution, surface appearance, and grain size are known to be important factors in determining the effectiveness of coatings used in the production of printed circuit boards. Square wave unipolar pulse and bipolar pulse reverse current electroplating of copper using an acid copper electrolyte has been reported to provide a coating superior to that produced by dc under certain pulsing conditions. The present paper describes a systematic assessment of the properties of electrodeposited coatings produced under unipolar pulse and bipolar pulse reverse current plating conditions. Results show that the overall coating uniformity under pulsing conditions is better than that obtained using dc plating. The surface texture of the coatings is shown to be strongly dependent on the pulsing conditions and is especially affected by bipolar pulse reverse conditions.

Rolling wear of TiN coatings induced by non-uniform Hertzian stress distribution

Abstract: The wear of TiN coatings in dry rolling contact with a non-uniform Hertzian stress distribution has been studied. Wear behaviour was observed in situ and the mechanisms of damage were examined by optical and electron microscopy and laser profiling. Changes of friction coefficient, weight, residual stress, percentage of undamaged TiN, and surface roughness were also observed. The failure mechanism of TiN coated SAE 52100 steel running against steel rollers was dominated by microcracking, transfer of material, and fatigue; that of TiN against TiN (both on M2 tool steel substrates) was principally adhesive failure, which occurred at the near surface carbides in the steel. Tests involving TiN coated rollers running against TiN coated rollers lasted 39 800 rolling contact cycles, whereas TiN versus steel failed after 8600 cycles and steel versus steel after less than 1700 cycles.

Neural networks in surface engineering

Abstract: (1995). Neural networks in surface engineering. Surface Engineering: Vol. 11, No. 2, pp. 89-90.